Pakistan is one of the remaining corners of the world where polio still lingers. Last year, the government declared a national emergency, and with the help of international institutions, embarked on an aggressive vaccination campaign.

So far, the results have been promising. The number of new polio cases is about a third of last year's total of 198.

But the new campaign, like previous efforts, hasn't been able to overcome one critical problem: getting into parts of Pakistan's lawless tribal regions along the border with Afghanistan to vaccinate the children there.

About 75 percent of Pakistan's polio cases can be traced back to certain areas there, primarily FATA, the Federally Administered Tribal Areas, and neighboring Khyber Pakhtunkhwa. Quetta, in Baluchistan, is also difficult to enter, says Pakistan's point person for polio, Shahnaz Wazir Ali.

The Taliban, al-Qaida and other Islamist groups are based throughout the region, and it's volatile and dangerous.

Khan Wali, 21, scrutinizes passing cars for young children at the traffic checkpoint. If a child appears to be younger than 5, the driver must pull over.

"This is a traditionally tribal society," Wazir Ali says. "And no one from outside can access those communities because of their very, very strict tribal laws."

"In certain areas, we have not been able to access children for more than three years because of military operations, because of the American drone strikes ... because of the reaction to what the Americans and other countries are doing in Afghanistan," Wazir Ali says.

In July, the Taliban flat-out banned any polio teams from entering North and South Waziristan until the U.S. drone attacks stopped.

Rumors about the polio vaccine are rife: It'll make the children sterile; it contains the AIDS virus; the vaccinators are really CIA agents.

Plus, it's difficult enough getting into these areas. And the population — mostly ethnic Pashtuns — is highly mobile. People often leave the tribal area to escape the danger or to find work, and they take the polio virus with them.

Wali marks a child's finger with ink after administering the polio vaccine. Children need to be inoculated multiple times in order to be fully immunized against the virus.

Checkpoints have also been set up on major thoroughfares and at railway stations across the country to catch the children from the Pashtun areas, who would not otherwise be inoculated.

At one checkpoint outside of Islamabad, 21-year-old Khan Wali scrutinizes every car that goes by to see if there is a child under 5. Police officers stand next to him to make sure the car stops.

Wali asks parents if their babies have been vaccinated, and he checks the child's finger. If there's a blue mark on the nail, then the child has been recently inoculated. If not, Wali pulls a vial out of a blue cooler and tips the vaccine into the child's mouth. The whole thing takes less than a minute.

Many children are malnourished and aren't very healthy, so their immune systems are very weak. To be fully protected from polio, the children will need several doses of the vaccine.

There are already 10 polio campaigns scheduled each year, yet Elias Durry, who leads Pakistan's polio eradication program for the World Health Organization, says he knows they're missing so many children.

Social mobilizers like Awaiz Amjad (right) walk through dense slums and go door to door inoculating children. Many children are malnourished, with weak immune systems.

"When you look into different parts of the country, each time we have, like, 1 million children at one point that we are not able to access," Durry says.

And for every child that exhibits polio paralysis, there are more than 100 carrying and transmitting the virus.

Health workers monitor the progress of vaccination campaigns by looking for the virus in community sewages.

Far fewer areas are testing positive for polio, as the international campaign in Pakistan takes effect — but not the city of Rawalpindi. Pakistan's fourth-largest city, which sits right next Islamabad, is considered a high-risk area for polio, and it usually tests positive.

Still, there have been only three cases of polio discovered in the city over the past couple of years. Recently, a 3-year-old Pashtun boy was diagnosed here. After that, teams of vaccinators spread through the Pashtun slums in Rawalpindi and neighboring Islamabad.

Amjad checks a child's fingernail to see whether he has been inoculated in a Lahore slum. It can be dangerous work: Some vaccinators have been attacked, and one was killed in a Karachi slum this summer.

The slums are congested and filthy. There's no sanitation, and the children are in rags. The vaccinators go house to house, looking for children and delivering extra doses of the polio vaccines.

They examine the writing left by vaccinators on an old wooden door. And they double-check the children's fingernails for the telltale mark that they were given the additional doses. Then they move on to the next house, scrupulously tracking down every child.

The vaccinators are three women with their faces covered with veils. They carry a cooler containing the polio vaccine.

They say many of the mothers ask a similar question. "They are asking questions repeatedly: 'Why are you again and again administering the polio drops?' " Khadeja Kubra, who works for the health department, says. "We have to tell them there is a polio case in Rawalpindi and we will, inshallah, come back in a week."

It can be dangerous work. Some vaccinators have been attacked. Another was killed in a Pashtun slum in the southern city of Karachi this summer.

Health care workers take samples of sewage water to check for the polio virus in Rawalpindi, a city near Islamabad known to be at high-risk for polio.

UNICEF has trained and recruited more than 1,000 special health workers to educate families about the polio virus, encourage people to vaccinate their children, and dispel rumors about the vaccine. These so-called social mobilizers talk to students, women's groups and, in particular, religious leaders, who have an enormous influence in the community.

It's a painstaking job, but the health workers know they have to repeatedly vaccinate every single one of Pakistan's 34 million children if the country wants to eradicate polio once and for all.

Chasing Down Polio:

Transcript

MELISSA BLOCK, HOST:

This is ALL THINGS CONSIDERED from NPR News. I'm Melissa Block.

ROBERT SIEGEL, HOST:

And I'm Robert Siegel.

In 2011, Pakistan had more cases of polio than any other country in the world, 198. Late last year, Pakistan's government declared a national emergency. It then embarked on an aggressive campaign to eradicate the virus, with help from a number of powerful institutions, including the World Health Organisation and the Bill and Melinda Gates Foundation. Beating polio in Pakistan required new strategies. And so far, the results are promising.

But NPR's Jackie Northam reports that some endemic problems remain.

JACKIE NORTHAM, BYLINE: When the international community swooped in to help Pakistan, it erased many of the problems that had plagued the government's earlier efforts: corruption, inefficiency, mismanagement. Things had got so bad at one point, seven and eight-year-olds were sent out to vaccinate children just a few years younger than themselves.

But as part of a new national campaign, the WHO, UNICEF and others streamlined the eradication effort and one-quarter of a million trained vaccinators fanned out across the country. So far, the results have been promising. The number of new polio cases is about a third of last year's total. But the new campaign, like earlier efforts, has been unable to overcome one critical problem - that is getting into parts of Pakistan's lawless tribal regions.

Shahnaz Wazir Ali, Pakistan's point person for polio, says the area that runs along the border with Afghanistan is incredibly difficult to access.

SHAHNAZ WAZIR ALI: First of all, the terrain is probably one of the roughest in the world. It is rugged. It is mountainous. It's craggy. You do not have roads and pathways everywhere. This is a traditionally tribal society and no one from outside can access those communities because there are very, very strict tribal laws.

NORTHAM: Wazir Ali says about 75 percent of Pakistan's polio cases can be traced back to certain areas of the tribal region, which is a base for the Taliban, al-Qaida and other Islamist groups. It's a volatile, dangerous area.

ALI: In certain areas we have not been able to access children for more than three years. For three - because of military operations, because of the American drone strikes, because of the reaction to what the Americans and other countries are doing in Afghanistan.

NORTHAM: In July, the Taliban flat-out banned any polio teams from entering North and South Waziristan until the U.S. drone attacks stopped. There are wild rumors about the polio vaccine - it'll make the children sterile, it contains the AIDS virus, and many believe vaccinators are CIA agents.

It's hard enough getting into parts of the tribal region, but the population - mostly Pashtuns - are also highly mobile, often leaving the tribal area to escape the danger or to find work, and taking the polio virus with them.

(SOUNDBITE OF TRAFFIC)

NORTHAM: Checkpoints have been set up on major thoroughfares and at railway stations across the country. At this one, outside of Islamabad, 21-year-old Khan Wali scrutinizes every car that goes by, to see if there's a child under five. Police officers stand next to him to make sure the car stops.

Wali asks the parents if the baby has been vaccinated and he checks the child's finger. If there's a blue mark on the nail, that means the child has been recently inoculated. If not, Wali pulls a vial out of a blue cooler and tips the vaccine into the child's mouth. The whole thing takes less than a minute.

Elias Durry, who leads Pakistan's polio eradication program for the WHO, says this is a good way to catch those children who would not otherwise be inoculated. Many children are malnourished and aren't very healthy, so their immune system is very weak and they will need several doses of the vaccine. There are already 10 scheduled polio campaigns a year. And yet, Durry says he knows they're missing so many children.

ELIAS DURRY: When you look into different parts of the country, almost each time we have almost a million children at one point that we are not able to access.

NORTHAM: And for every child that exhibits polio paralysis, there are more than a hundred carrying and transmitting the virus.

(SOUNDBITE OF RUNNING WATER)

NORTHAM: A stream of muddy-looking water flows between steep, garbage-strewn hills in the city of Rawalpindi, next to the capital Islamabad. The stench is enough to make your eyes water.

DR. PERVEZ YOUSAF: It's an open sewerage. We don't have the sewerage system. Before there was one pumping station in Rawalpindi but that's not working.

NORTHAM: Dr. Pervez Yousaf, with the WHO, says once a month the fetid sewage water in Rawalpindi is tested for the polio virus. He watches as a health worker rolls up his trouser legs and heads down the steep hill with a bucket in his hand to collect a sample. Rawalpindi is considered a high-risk area for polio.

YOUSAF: We have taken the samples from the sewerage and now we are packing it in the special bottle...

NORTHAM: The sample is sent to a nearby lab by motorcycle. As the international campaign in Pakistan takes effect, far fewer areas are showing the polio virus in the sewage water. But not Rawalpindi - it usually tests positive for the virus. Still, there have only been three full-blown cases of polio discovered in the city over the past couple of years. Recently, a three-year-old Pashtun boy was diagnosed here. After that, teams of vaccinators spread through the Pashtun slums in Rawalpindi and neighboring Islamabad.

(SOUNDBITE OF CHILDREN)

NORTHAM: The slums are congested and filthy. There's no sanitation and the children are in rags. The vaccinators go house to house, looking for children, and delivering extra doses of the polio vaccine.

KHADEJA KUBRA: This shows the team has visited this house and administered drops to children.

NORTHAM: Health care workers examine the writing left by vaccinators on an old wooden door. It details how many children are there and whether they were inoculated. But the health care workers need to double-check that no child was missed.

(SOUNDBITE OF KNOCKING)

NORTHAM: Khadeja Kubra, with the health department, checks if all the children in this compound were given the additional dosage.

UNIDENTIFIED WOMAN #1: (Foreign language spoken)

KUBRA: (Foreign language spoken)

NORTHAM: Both children have the telltale mark on the fingernail. Kubra moves on to the next house. After a short while, she catches up with a team of female vaccinators.

Three woman, their faces covered with veils, carry a cooler containing the polio vaccine. It can be dangerous. Some vaccinators have been attacked; another was killed in a slum in the southern city of Karachi this summer. These women say they have had no problem. They say many of the mothers ask a similar question.

UNIDENTIFIED WOMAN #2: (Through Translator) They are asking questions repeatedly: why are you again and again administering the polio drops? We have to tell them there is a polio case in Rawalpindi and we will, Inshallah, come back in a week.

NORTHAM: Tribal region, on a hilltop or under a bridge, the health care workers have to routinely vaccinate every single one of the country's 34 million children, if Pakistan wants to eradicate polio once and for all.